Din Oring pops out all the time when somebody tries to use a standard 112 oring instead of the 111 it needs. Keep spares of the correct Oring on hand. A 112 will fit in a pinch, but it will come out every time you take the reg off the tank.
First stage seats used to be a huge issue, haven't seen one in a few years as they changed the kit without changing the part number to skate out of any recalls or responsibility.
If it has been serviced in the last year or two by a shop that does any volume, you are good to go.
Second stages don't have any issues on xtx line outside of the lock tabs are somewhat flimsy for the barrel. A hamfisted tech can easily break them during service.
The old first generation TX second stages had a main diaphragm with the wrong material used. I doubt there are too many of those diaphragms left to find at this point.